The invention relates to an optical displacement measuring system using a PSD, position sensitive device.
Optical displacement measuring systems using PSDs are known in the art, for example Hamamatsu Catalog APR/85. A PSD has a pair of outputs providing electrical current I1 and I2, FIG. 1, satisfying the equation ##EQU2## where x is the position of a spot of infrared light along a surface of the PSD reflected by a target from a light source, which in turn gives the target distance by triangulation. For example, target 1 at distance d1 provides a reflected spot at position x1, FIG. 1, and target 2 at distance d2 provides a reflected spot at position x2. When the spot is in the center, I1=I2. As the spot moves toward the I2 terminal, the current I2 increases and the current I1 decreases. I1 and I2 are measured, and the sum I1+I2 and the difference I1-I2 are calculated by a computer or the like to solve the position equation for x. Alternatively, the sum I1+I2 is held constant, to normalize the denominator of the fraction in the above noted position equation, see Japanese Patent laid-open No. 13412-1989, and also see Japanese Patent laid-open No. 44809-1982. For a short range PSD, e.g., a target distance of about 0.05 inch to 0.25 inch, the light source is continuously on. For a long range PSD, e.g., a target distance up to about 4 inches, the light source is pulsed at a given duty cycle to prevent burnout due to the higher intensities required for longer range transmission.
The present invention provides an optical displacement measuring system with a long range PSD and a particularly simple and effective system for normalizing the denominator in the above noted position equation. The invention also provides improvements in linearity compensation, ambient light compensation, and out-of-range indication.